Sci-ber Text - 8th Grade

Force, Energy, & Motion - Modeling Heat Movement

Heat is in constant motion. It flows from warmer areas to colder areas until an overall equal temperature is established. Heat moves from one place to another in three ways. The movement of heat from a warmer object to a cooler one is called heat transfer. There are three methods of heat transfer: conduction, convection, and radiation.

Conduction is the movement of heat energy through a substance or from one substance to another by direct contact of atoms and molecules. Heat moves directly from one molecule to another. The heat energy speeds up the movement of the atoms and they collide with other molecules setting them into faster motion. This goes on until all the molecules are moving around faster and the entire object becomes hot. What you feel is heat moving along the object from the source to the end. Have you made a cup of hot chocolate? As the hot chocolate sits in the mug the mug begins to get hotter. If you leave a metal spoon in the cup it will get hotter too. Where will the spoon get hot first? Where will it be the coolest? What will happen to the spoon after some time if it is left in the cup? Can you think of other examples of conduction? Think about heat transfer through another substance.

Convection is the transfer of heat in a liquid or gas as groups of molecules move in currents from one region to another. Warm air is less dense than cooler air. The less dense air rises while the cooler more dense air sinks. The heavier, colder air sinks and pushes the lighter, warmer air up. This circular movement is called a convection current.

Hot air balloons use this principle to lift off the ground. The air in the balloon is less dense than the surrounding air and rises to lift the balloon off the ground.

Water is also heated by convection. When water is heated in a pan, the water near the burner is heated first. The heat causes the water to expand, it becomes lighter, and it rises to the top of the pan. The heavier, colder water near the top sinks down, pushing up the hot water. This continues until all the water is evenly heated. You can see the path of a convection current if you watch spaghetti cooking in a pot of boiling water. The spaghetti will move in a circular path around the edge of the pot as the water boils.

Radiation is heat energy transmitted through empty space or heating the empty space by electromagnetic waves or infrared rays. This process involves only the molecules of the substance radiating the heat. Radiation can occur through a vacuum and is the way the sun's heat is transferred to Earth. Heat energy is sent to Earth as waves from the sun. These waves travel through space and the atmosphere of Earth. When the heat waves hit an object on Earth, the energy passes into the object, and it becomes heated.

Other familiar forms of heat transfer by radiation include the heat you can feel around an open fire or a candle flame, the heat near a hot stove, and the heat given off by an electric heater.

You can model the movement of molecules as heat energy moves in the following activity.

Materials:

Inflated ball
Chairs
Group of friends

Procedure:

One person represents the original heat source and stands at one end of the room.

The goal each time is to roll the inflated ball (heat energy) to a friend standing at the opposite end of the room.

Trial #1: Have nothing in the way between the heat source and the person getting the heat energy.

Is there too much of a challenge in getting the inflated ball (heat energy) to the person getting the heat energy?

Trial #2: Have other people line up between the heat source and the person getting the heat energy.

Each person in the line passes the ball to the person behind him/her.

Trial #3: Move other objects (chairs or desks) between the heat source and the person getting the heat energy.

Safety concerns: Be sure to follow all eye safety rules that are specified by your teacher in all general laboratory experiences. As with all science lab activities, the most important safety rule is to follow all teacher directions.

Analysis:

Which method of moving the heat energy was the easiest?

What form of heat transfer did it represent?

Which method of moving the heat energy was the most difficult?
- What form of heat transfer did it represent?

Highlight the box below to check your answers.

1. Trial #1 - it represented radiation.

2. Trial #2 - it represented conduction.